The present invention relates to a process for the production of stable prepolymers and the production of polyurethane elastomers from such prepolymers.
Prepolymers are NCO-group-terminated polymers that are obtained by reacting a polyol with a polyisocyanate in molar excess, based on functional groups, at a temperature of from room temperature to about 100° C. (in special cases also over 100° C.) until a constant NCO value is reached.
An important application for such NCO-terminated prepolymers is the production of elastomers by the casting process. In the production of such elastomers, the prepolymer either undergoes chain extension immediately after production (i.e. reaction, with a short-chain polyol (e.g., 1,4-butanediol) or with a polyamine (e.g., methylene bis(orthochloroaniline)) or with water), or the NCO prepolymer is cooled (to the extent that it is advantageous and possible) to a lower temperature (storage temperature) for the purpose of a subsequent chain extension and stored.
Elastomers which are produced with prepolymers on the basis of high-melting polyisocyanates exhibit better properties than those which are based on low-melting polyisocyanates or those that are liquid at room temperature, such as toluene diisocyanate (TDI) or diphenylmethane diisocyanate (MDI), for example.
Cast elastomers can be obtained in principle not only by the prepolymer process but also by the one-shot process in which a mixture of long-chain and short-chain polyols is reacted with one or more polyisocyanates. The disadvantage of the one-shot process, however, is that only low-grade elastomers are obtained, especially if high-melting polyisocyanates are used, because intermediates formed by short-chain polyol (chain extender) and high-melting polyisocyanate are precipitated out of the reaction melt in some cases and therefore undergo a further reaction, preventing further ordered molecular weight development. This is one reason why the prepolymer process normally leads to better products.
Another favorable feature of the prepolymer synthesis route is that a portion of the heat of reaction is already removed by the prepolymer step, so the exothermic heat of reaction generated during the actual polymer formation process is smaller. This has a favorable effect on the speed of molecular weight development and allows longer casting times—a processing advantage.
The prepolymer synthesis route is particularly advantageous for MDI-based systems (melting point of the 4,4′-isomer approx. 42° C.) because the polyisocyanate melting point is lowered with prepolymerization. MDI may be brought to a form that is liquid at room temperature through prepolymerization (i.e., formation of an NCO prepolymer), which naturally makes processing significantly simpler in comparison to the solid form. MDI prepolymerization also slows the undesirable dimerization of the monomeric polyisocyanate down markedly.
U.S. Pat. No. 6,515,125 teaches that partially trimerized polyisocyanate prepolymers of toluene diisocyanate (TDI) or diphenylmethane diisocyanate (MDI) are stable in liquid form without formation of solids.
It is also possible to incorporate allophanate groups in MDI-type NCO prepolymers (U.S. Pat. No. 5,440,003), as a result of which storage stability can likewise be achieved in the liquid state at 25° C. This is achieved in a manner analogous to that disclosed in U.S. Pat. No. 4,738,991.
However, neither trimerization nor incorporation of allophanate groups can be used for a high-melting polyisocyanate such as NDI because unlike MDI, NDI displays a markedly lower tendency towards trimerization and allophanate groups incorporated in NDI-based systems lead to a very sharp rise in viscosity.
A number of possibilities for providing MDI at room temperature in the form of stable, liquid NCO prepolymers are also described in the literature. In U.S. Pat. Nos. 4,115,429 and 4,118,411 this object was achieved by reacting (a) specific amounts of 2,4′-isomers mixed with 4,4′-isomers with (b) propylene glycol or poly-1,2-propylene glycol. However, in glycol-extended systems, the use of mixtures of isomers leads to elastomers having inferior properties. The same applies to mixtures of isomers of phenylene diisocyanate. The use of mixtures of isomers has to be ruled out for NDI, however, due to lack of commercial availability.
In U.S. Pat. No. 4,490,300, the crystallization of MDI is prevented by reaction with a diol carrying bulky groups, such as 2-methyl-2-phenyl-1,3-propanediol or phenyl-1,2-ethanediol. The disadvantage of this approach, however, is that the rigid segments are formed in a less ordered manner resulting in a deterioration in properties, which for NDI-based polyurethane (PU) elastomers is unacceptable due to the extremely demanding applications for which they are used.
In the case of MDI, a lowering of the melting point by from 15° C. to 40° C. to from 0° C. to 25° C. can be achieved, e.g., by reaction with a diol. The object is thus satisfactorily achieved for MDI-containing systems and is also used in industry.
In the prepolymer process, however, a whole series of boundary conditions are critical and must be observed. These are, in particular, the storage stability of the prepolymer and the viscosity of the prepolymer. In addition, as mentioned above, the measures taken to liquefy the polyisocyanate, i.e. lowering the polyisocyanate melting point, must not adversely affect the PU properties in the particular applications to any great extent.
It is desirable for NCO prepolymers to be stable at the storage temperature, i.e. as far as possible for no secondary reactions to take place and for the viscosity to change only slightly over time and remain within the processing window.
In some systems, moreover, high concentrations of free monomeric polyisocyanate are undesirable due to toxicological problems. The free monomeric polyisocyanates can largely be removed from the prepolymer by thin-film or short-path evaporation. This procedure is very expensive, however, and also leads to elastomers having inferior properties because the length of the rigid segments remains restricted to building blocks made up of only two diisocyanate molecules and one chain-extender molecule.
In the case of NDI, however, not only is this measure not absolutely necessary because of its higher boiling point in comparison to TDI and para-phenylene diisocyanate (PPDI), for example, but it can only be implemented on an industrial scale at significantly increased cost, even though it would solve the problem of sedimentation stability as discussed below.
Prepolymers having a relatively low content of free polyisocyanate can be produced comparatively easily, particularly if polyisocyanates having differently reactive NCO groups are used, because the more reactive NCO group in each case is preferentially attached to the polyol. Examples of such prepolymers are NCO prepolymers based on 2,4-toluylene diisocyanate or isophorone diisocyanate (IPDI). Further, the differently reactive NCO groups also allow the stoichiometry of NCO to OH groups to be reduced to well below 2:1, because the increase in viscosity is restricted to a minimum because of a pre-extension of the polyol.
In the case of 1,5-NDI, the NCO groups are practically identical in terms of their reactivity. Therefore, if the stoichiometry is reduced below 2:1, the proportion of free monomeric polyisocyanate in the NCO prepolymer is smaller, albeit at a higher level than with analogous 2,4-TDI and 2,4′-MDI systems, but the extent of pre-extension and resultant increase in viscosity are much more apparent. A sharply increased viscosity is very disadvantageous for PU applications. An elevated viscosity is a considerable processing disadvantage, so the stoichiometry of NCO and OH groups, particularly in the case of NDI-based prepolymers, must be chosen with special care.
It is also known that the viscosity of NCO-prepolymers based on polypropylene glycols is lower than that of NCO prepolymers based on polyadipate polyols under otherwise identical conditions. The chemical structure of the structural components has an evident effect on the properties.
It is also known that the viscosity of the prepolymer can be kept low if the polyisocyanate is used in large molar excess.
NCO prepolymers based on TDI or MDI, for example, are produced by preparing the liquid, optionally molten, polyisocyanate and slowly adding the polyol. This procedure ensures that at any stage of the reaction, the NCO groups are in excess in comparison to the hydroxyl groups. This measure prevents pre-extension of the polyol and has a favorable effect on the viscosity of the NCO prepolymer. This procedure cannot be used for NDI prepolymers, however, because the use of molten polyisocyanate would in this case mean that the reaction would have to be performed above the melting point of NDI (approx. 127° C.), which in addition to all the plant engineering challenges would also mean that all reaction paths leading to viscosity-raising structural elements would be open to the developing NCO prepolymers for some time.
It is clear from the discussion above that prepolymers based on high-melting polyisocyanates, such as NDI, present a very particular technical challenge. It would be desirable to remove (e.g., by distillation methods) the high-melting polyisocyanates which as monomers have a particular tendency to crystallize out of prepolymers. Such removal is either not feasible or feasible only with considerable technical difficulty because of the high boiling point associated with the high melting point of such polyisocyanates. The measures suitable for other polyisocyanates, such as varying the stoichiometry, incorporating crystallization-inhibiting additives, etc., are impossible or possible only within limits for the reasons already specified.
The production of useable and stable NCO prepolymers based on NDI is therefore unsuccessful with the aforementioned measures.
As already stated above, NCO prepolymers which have not been subjected to a thin-film or short-path evaporation stage, always contain monomeric, unreacted polyisocyanate molecules. In the case of NDI, they are characterised by poor solubility in the NCO prepolymer, such that at low temperatures they crystallize out. Further, due to its high density, crystallized high-melting polyisocyanate settles at the bottom of the storage vessel. This means that the storage containers would also have to be stirred with a bottom-driven impeller. Not only are sediment-containing prepolymers unsuitable for the chain-extension reaction because extension cannot take place, or cannot take place to a sufficient extent, but they also always present the risk of blockages in pipes and machines. Until now, such NCO prepolymers have therefore had to undergo further processing immediately after production, since they allow only extremely short storage periods at the necessary high temperatures.
Additionally, the NCO prepolymers based on NDI cannot be converted to the liquid state by heating without a corresponding negative effect on the viscosity and the NCO content. For the purposes of homogenization prior to chain extension, prepolymers based on high-melting polyisocyanates would have to be heated to an adequate temperature either to dissolve or to melt solid polyisocyanate in the NCO prepolymer. This process is not only cost-intensive but due to the elevated temperature it also leads to an increase in secondary reactions and hence to a sharp rise in viscosity and a reduction in the NCO content, such that the prepolymers are or become unusable.
These problems, in particular in relation to NDI prepolymers, are summarized in “Solid Polyurethane Elastomers”, P. Wright und A. P. C. Cummings, Maclaren and Sons, London 1969, page 104 et seq./chapter 6.2, as follows:
6.2.1. Unstable Prepolymer Systems (Vulkollan)
Vulkollan is manufactured by a prepolymer route, although the prepolymer is non-storable and must be further reacted within a short interval of time . . . . The prepolymer so formed is relatively unstable since further undesirable side reactions can take place. To reduce the possibility of these side reactions occurring, the next stage in the process, viz. the chain extension, should take place as soon as possible but within a maximum of 30 minutes.”
In view of the above-discussed problems and the fact that there are no commercially available NCO prepolymers produced from NDI and that as a consequence the advantages of preproduced NCO prepolymers in the production of polyurethane cast elastomers have, until now, had to be forgone, gives rise to the object of the present invention.